Render turns white on completion
In the compositor you have plugged the 'Alpha' channel into the 'Image' output and the 'Z' channel into the 'Alpha' output. The image doesn't yet have an alpha channel (you just need to enable 'Transparent' under the 'Film' section of the 'Render' tab) and so the final image appears blank.

It looks like you may have just accidentally connected the alpha channel to the output, rather than wanting it, to output a normal image the connections should be: